Binuclear non-heme iron active sites are present in a wide variety of proteins and enzymes which perform different biological functions. This proposal will specifically investigate those sites present in sMMO, R2, D9D and frog M-ferritin to elucidate their differences in dioxygen reactivity (i.e. hydroxylation, radical generation, desaturation and ferroxidation). These differences have led to the identification of various diiron peroxy (P) and high-valent (Q and X) intermediates. While these enzymatic intermediates have been investigated, relatively little is known about their geometric and electronic structure and, more importantly, how their structure relates to reactivity. Therefore, the major objectives of this research proposal are to use biophysical spectroscopic techniques (EPR, resonance Raman and UV-vis/Near IR absorption/CDNTVH MCD) to characterize the geometric, electronic and magnetic properties of the various diiron peroxy (P) and high-valent (Q and X) intermediates in these enzymes and relevant model complexes and apply computational methodologies to gain greater insight into the mechanistic conversion between these intermediates.